1. Field of Invention
This invention relates generally to contraband detection systems and more specifically to registration of images in a contraband detection system.
2. Discussion of Related Art
Contraband detection systems are widely used in settings where it is desirable to determine whether an item conceals contraband objects. A prevalent use of such systems is in the detection of weapons or explosives inside suitcases or packages before those items are allowed on airplanes. However, contraband detection systems are used in other facilities where it is desired to prevent weapons or explosives from entering a secured area. Additionally, contraband detection systems are used to identify objects other than weapons or explosives. For example, such a system may be used to check for drugs, illicit currency or other contraband hidden in parcels or other objects.
Frequently, contraband detection systems detect contraband by processing an image of an item under inspection. The image is formed by measuring penetrating radiation that has passed through, and been attenuated by, an item under inspection. The image represents objects inside the item and contraband is frequently detected by identifying regions in the image representing an object of a shape or density characteristic of a contraband object.
Other information may be used to detect contraband objects hidden in items under inspection. For example, some contraband detection systems image items using radiation of multiple energy levels. Because the amount an object attenuates radiation depends on the object's atomic number, multi-energy radiation that has passed through an item allows an effective atomic number of objects in the item to be computed. Identifying regions in the image that represent objects having an effective atomic number characteristic of contraband can increase the accuracy with which contraband is detected.
Various imaging configurations are used in contraband detection systems. In some configurations, the system may acquire an image using radiation passing through an item under inspection in a single direction. Such a configuration results in a projection, or “single view,” image, which contains a two dimensional representation of objects in the item under inspection. A two-dimensional image is often useful for contraband detection, particularly when contraband objects of interest have a distinctive appearance regardless of the angle from which they are viewed.
Multi-view imaging systems are also used. In a multi-view system, radiation passes through an item under inspection from multiple directions, such as from three directions. This information can be processed to compute an “image” that more accurately represents objects inside the item.
An image that even more accurately represents an item can be formed with a volumetric imaging system. In a volumetric imaging system, an image of an item is created as a set of “voxels.” Each voxel represents properties of a small volume of the item under inspection. Accordingly, the contraband detection system identifies contraband objects by finding groups of voxels with characteristics indicating that a contraband object is present. As with projection or multi-view imaging systems, a volumetric image can represent mass, atomic number or other characteristics of an item under inspection.
Volumetric images are frequently formed using computed tomography (CT) processing. Traditional CT processing builds a volumetric image by measuring characteristics of thin sections, called “slices,” of an item under inspection. The system obtains data on a slice that is adjacent an x-ray source that moves around the item. Once sufficient data has been gathered for one slice, the system moves the item under inspection so that another slice is adjacent the source. Data from slices is grouped to form a representation in three-dimensions of the item under inspection.
Some systems use a related technique called a helical scan. In a helical scan, the item under inspection moves continuously past the x-ray source as data is collected. Mathematical manipulation of the data is used to create a volumetric image comparable to that created with a traditional CT scanner.
Regardless of the specific type of system used to form an image, it is desirable that the identification of contraband objects be quick and accurate. Automated processing is frequently used to identify suspect objects within an item. Initial processing may not definitively identify whether an item contains a contraband object, but may be sufficient to distinguish between an item that warrant further inspection and items that are “cleared” and require no further inspection. This initial processing is sometimes referred to as Level I inspection.
Items deemed suspicious at Level I inspection are then passed to Level II (or higher levels) inspection. Level II could involve further automated inspection or may involve manual inspection.
Various systems using multiple levels of inspection are known. Examples of such systems are provided in U.S. Pat. No. 5,642,393 entitled “Detecting Contraband by Employing Interactive Multiprobe Tomography,” by Krug et al. and U.S. Pat. No. 6,856,667, entitled “X-Ray Inspection System” by Ellenbogen, both of which are hereby incorporated by reference.